This invention pertains to a composite turbo-fan and turbo-shaft engine which is capable of operating in either a thrust generating mode or a shaft horsepower mode or combinations of both output modes. An engine which is convertible between thrust and shaft horsepower modes may be used to power the rotor of a rotary wing aircraft during liftoff; then, once the craft is airborne, gradually may be switched to its thrust mode for forward flight. Also, in turbo-fan powered multi-engined aircraft, which make use of powered lift concepts, a selective redistribution of shaft power to the fans of such engines on opposite sides of the wing may be used to effect roll control of the aircraft at low flight speeds when conventional aerodynamic control surfaces may be ineffective.
Three specific operational conditions exist with a composite turbo-fan and turbo-shaft engine namely, one hundred percent propulsive thrust, one hundred percent shaft power output, and a split between the two. A constant output shaft speed is generally required in rotary wing applications. Therefore, in order to accomplish the various modes of operation with both the propulsion fan and shaft output powered by the same turbine, a mechanism is required which will provide a smooth transition between operating modes and a means to balance the load distribution between turbo-fan and turbo-shaft outputs. Generally, the balancing may be achieved by load control over both the fan and shaft output. In rotary wing aircraft, the rotor flight control system, namely collective pitch, will provide this function. In the prior art cited herein, the fan may be rigidly connected to the gas turbine shaft operating at constant speed, and fan power requirements are usually controlled by aerodynamically unloading the fan by various expensive devices, for example: variable inlet and outlet guide vanes, blade pitch change, selective direction or splitting of the hot gas stream to turbines dedicated to the different functions. In this invention, however, fan power absorption is regulated by controlling the fan speed of a fixed pitch fan by means of the relatively more economical infinitely variable speed-ratio hydrodynamic torque converter transmission. This mechanism provides a very flexible way of selecting a variety of modes and combinations of modes with a smooth transition between them while driving the turbo-fan and turbo-shaft outputs with the same turbine.